Steam cracking, also referred to as pyrolysis, has long been used to crack hydrocarbon feedstocks into a wide range of relatively high value molecules, including ethylene, propene, butenes, steam cracked gas oil (“SCGO”), etc. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value heavy products, such as pyrolysis tar. When the pyrolysis is produced by steam cracking, the pyrolysis tar is identified as steam-cracker tar (“SCT”). Economic viability of refining processes relies in part on the ability to incorporate as much of the product and residual fractions, such as SCT, into the value chain. One use of residual and/or relatively low value products is to blend these fractions with other hydrocarbons, e.g., with other feedstreams or products.
SCT, however, generally contains relatively high molecular weight molecules, conventionally called Tar Heavies (“TH”). The presence of TH renders the SCT incompatible for blending, e.g., with fuel oil blend-stocks or different SCTs. Compatibility is generally determined by visual inspection for solids formation, e.g., as described in U.S. Pat. No. 5,871,634. Generally, SCTs are not compatible with other heavy hydrocarbons such as fuel oil, or are only compatible in small amounts. Likewise, SCTs produced under specific conditions are generally incompatible with SCT produced under different conditions.
Compatibility can be improved by treating the SCT with a gas containing molecular hydrogen in the presence of a hydroprocessing catalyst, typically containing one or more of Co, Ni, or Mo. Catalytic hydroprocessing of undiluted SCT, however, leads to significant catalyst deactivation and the formation of undesirable deposits (e.g., coke deposits) on the reactor internals. As the amount of these deposits increases, the yield of the desired upgraded pyrolysis tar (upgraded SCT) decreases and the yield of undesirable byproducts increases. The hydroprocessing reactor pressure drop also increases, often to a point where the reactor is inoperable.
It is conventional to lessen deposit formation by hydroprocessing the SCT in the presence of a fluid, e.g., a solvent having significant aromatics content. The product of the hydroprocessing comprises an upgraded SCT product that generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the feed's SCT, resulting in improved compatibility with fuel oil blend-stocks. Additionally, hydroprocessing the SCT in the presence of fluid produces fewer undesirable byproducts and the rate of increase in reactor pressure drop is lessened. Conventional processes for SCT hydroprocessing are disclosed in U.S. Pat. Nos. 2,382,260 and 5,158,668; and in International Patent Application Publication No. WO 2013/033590, which involves recycling a portion of the hydroprocessed tar for use as the fluid.
Hydroprocessing, however, is a relatively costly method for upgrading SCT to make it more suitable for blending. Moreover, the benefits of hydroprocessing can sometimes be less than desired. For example, an SCT may require filtration or long settling periods before, during, or after hydroprocessing, in order to remove solid particles. Filtration may be needed even when the SCT is hydroprocessed in combination with a fluid. Solids removal by particle settlement can be extremely slow and/or energy intensive, leading to the presence of problematic molecules even after settling. Each of these methods adds costs and can still lead to an SCT that is incompatible with fuel oil, and to other hydroprocessing problems as described above.
Thus, a process that selectively removes problematic components from hydrocarbon feedstock such as SCT to improve the compatibility of hydrocarbons with fuel oil and/or renders the SCT more amenable to hydroprocessing would be beneficial.